(1) Field of the Invention
The present invention relates to an electronically controlled fuel injection type two-stroke engine.
(2) Description of the Related Art
Some types of engines use a fuel injection device for mixing intake air with fuel, instead of a carburetor. The fuel injection device directly injects the fuel into air in the intake air passage so as to deliver the air-fuel mixture to the engine.
A typical fuel injection device has a microcomputer as a central processing unit (CPU), which computes the amount of fuel to be injected based on the detection signals (data) from various sensors, and controls the amount of fuel injected from the injector by the output signal.
Since the exhaust port of a typical engine is open to the air, in computing the intake air amount of the engine, the basic air amount is calculated based on throttle opening (angle) and engine speed (revolution), and the calculated air amount is then further corrected based on the intake air temperature, the engine temperature, the coolant temperature, the atmospheric pressure, etc., to determine the fuel injection amount. This control system shows good throttle response.
Alternatively, Japanese Patent Application Laid-Open Hei 5 No. 18,287 discloses a system in which the exhaust pressure is sensed to correct the injected fuel amount.
However, in a case of an outboard motor engine used in a motor boat etc., the exhaust outlet of the engine is under water, the pressure of the exhaust passage is varied by the boat speed, the hull shape, the mounted position of the engine, running mode such as forward, backward and neutral, and the like. For such outboard motor engines as well as engines of which the exhaust system pressure is varied by some external factors, there are cases where the air amount computed based on the throttle opening and the engine speed and corrected based on the intake air temperature etc. differs from the air amount in an actual operation and this may cause troubles such as engine output lowering, degradation of fuel consumption and deterioration of the exhaust gas.
In contrast, in the techniques, as disclosed in the above publication, for correcting the fuel injection amount based on the exhaust pressure, disposing a sensor in the exhaust port requires a costly pressure-resistant and heat-resistant sensor and lowers the reliability of the sensor, because the temperature and pressure around the exhaust port are high. Further, the placement of the exhaust pressure sensor is difficult since the coolant passage and the scavenging port are arranged near the exhaust port.